This invention relates to ink jet printers. More particularly, it relates to the control of ink jet printers to optimize printing speed while permitting the printing of multiple sized images including fonts and graphics of various types.
A printhead optimized for high speed, small fonts will likely be unable to print tall character fonts. The reason is that the increased deflection required for tall fonts is usually achieved by lengthening the high voltage deflection plates. This also increases the distance to the substrate to be printed. If this distance becomes too great, accurate control of drop deflection becomes impossible for every drop printing. To correct for this the print drops are separated by uncharged guard drops and line speed is reduced by a factor dependent on the number of guard drops. Very often this line speed is lower than that obtainable by a printer optimized for the larger font. The insertion of guard drops to obtain increased deflection height and maintain quality lowers print drop frequency and therefore line speed. This results in a reduction in print line speed by digital steps (i.e. 1/2, 1/3, 1/4 by using every other drop, every third drop and every fourth drop to print).
A distance referred to as the "merge distance" defines the distance from the beginning of the deflection field to a point beyond which printing is no longer satisfactory, i.e. print quality line. There are merge distances for every drop printing, every other drop printing, every third drop printing and so on. These distances move out progressively since it is easier to achieve correct drop placement for every other drop printing than it is for every drop printing. The merge distance is largely determined by how accurately drops can be placed on a distant substrate. If the distance to the substrate is too great, distortions occur. These distortions are caused mainly by aerodynamic and electrostatic forces. If printing on the substrate is done within the merge distance, drops can be accurately placed. If printing occurs beyond this distance the drops cannot be accurately placed and distortion occurs.
It is important to optimize the print speed for both character definition and quality. Presently this is done by selecting the best frequency for a particular font and designing a printer for that font to the exclusion of significantly different sized fonts. Thus, tall, highly defined characters (requiring, for example, a 30.times.20 drops matrix) can be printed at their highest quality and speed by a first printer optimized to print them, while higher speed, smaller fonts (such as 5.times.7 drops) are printed by a different printer optimized to print the smaller fonts. The optimization of a printer for a particular font results in undesirable comprises. For example, to print tall fonts the deflection electrodes may need to be lengthened along with the use of guard drops, which then reduces printing speed. Another approach in the prior art is U.S. Pat. No. 4,337,470 to Furukawa. Drop size is varied to print large images using larger drops formed by reducing the frequency of the stimulation voltage applied to the nozzle stream. Smaller images are printed using smaller drops formed by increasing the frequency. This is undesirable because the quality of printing will vary if the drop size is varied.